I was playing around with pixel shaders that are included with DOSBox SVN Daum when I realized that several of them alter the DOSBox output to make it look like an image on an authentic CRT screen. One of them even goes as far as to recreate the "fishbowl" distortion of an actual monitor:

Another shader creates a less distorted (but still not completely rectangular) image:

In my opinion, this looks pretty awesome (especially the fishbowl effect one). To configure DOSBox SVN Daum to use this shader, you need to make the following changes in dosbox.conf:

The CRT-geom-curved.fx shader turns out to be quite customizable and allows to adjust screen curvature, gamma correction and other stuff, similar to an actual CRT screen. Detailed info can be found here.

I've asked the experts at VOGONS and it turns out that I made the mistake of taking screenshots in windowed mode, which does not do justice to the shaders' actual capabilities. leilei also suggested to use CRT.D3D.br.fx as the preferred shader.